1. Field of the Invention
This invention relates to an ink composition which is solid at ordinary temperature and melts upon heating, i.e., what is called a hot-melt solid ink composition. More particularly, it relates to a hot-melt solid ink composition which is stable to heat upon recording using ink-jet recording apparatus where ink is heated to melt at a temperature higher than ordinary temperature to make a record, and has a superior transparency and a superior adhesion to printing mediums.
2. Description of the Related Art
In conventional ink-jet recording systems, water-based inks employing water as a main solvent and oil-based inks employing an organic solvent as a main solvent are commonly used. Printed images obtained using the water-based inks have a poor water resistance in general. On the other hand, the oil-based inks can provide printed images having a good water resistance.
However, these water-based inks and oil-based inks are liquid at ordinary temperature, and so quickly permeate in recording paper when printed that feathering tends to occur and it has been difficult to attain a sufficient print density. Also, since the inks are liquid all the time, deposits such as aggregates of dyes tend to occur in ink during its storage, and such deposits may. e.g., stop up ink channels or ejection orifices to cause a great lowering of the reliability of ink-jet recording systems.
In order to overcome such disadvantages of the conventional inks of a solution type, what is called hot-melt oil-based ink-jet recording ink compositions, which are solid at ordinary temperature and melt upon heating, are proposed. Stated specifically, U.S. Pat. No. 3,653,932 discloses an ink containing a dialkyl sebacate. U.S. Pat. No. 4.390,369 and Japanese Patent Application Laid-open No. 58-108271 disclose an ink containing a natural wax. Japanese Patent Application Laid-open No. 59-22973 discloses an ink containing a stearic acid. Japanese Patent Application Laid-open No. 61-83268 discloses an ink containing an acid or alcohol having 20 to 24 carbon atoms and also containing a ketone having a relatively higher melting point than these. Japanese Patent Application Laid-open No. 62-48774 discloses an ink containing a thermosetting resin having a high hydroxyl value, a solid organic solvent having a melting point higher than 150.degree. C. and a small quantity of a dye substance. Japanese Patent Application Laid-open No. 2-167373 discloses an ink comprised of a colorant, a first solvent which is solid at ordinary temperature and capable of liquefying upon heating to a temperature higher than the ordinary temperature and a second solvent capable of dissolving the first solvent, being liquid at ordinary temperature and being highly volatile. Also, Japanese Patent Application Laid-open No. 62-295973 discloses an ink containing a synthetic wax having a polar group and a dye soluble in the wax.
These hot-melt solid ink compositions are set in hot-melt ink-jet recording apparatus, made liquid upon heating, ejected from ejection mechanisms in the liquid state, and applied on printing mediums to make prints. However, the conventional hot-melt solid ink compositions have had the problems that the ink itself may discolor when left in a molten state for a long time, and, especially in the case of color inks colored in vivid colors, the colors may blacken and the initial vivid colors can not be maintained, having a poor thermal stability. Such problems tend to become more serious as they have a higher melt temperature and are left for a longer time.
In hot-melt ink-jet recording systems in which such hot-melt solid ink compositions are used, the time after signals are sent at the time of printing and before the printing is started is desired to be as short as possible, and in recent years much more strongly so desired. Accordingly, even in the state of a stand-by of ink-let printers, it is desirable for the ink to stand heated and to have been heated up to ink ejection temperature, because images can be immediately printed as soon as the signals are sent. However, it is not desirable for the conventional hot-melt solid ink compositions to be left for a long time while they stand heated up to the ejection temperature, because the ink tends to discolor as stated above. Accordingly, in the conventional hot-melt solid ink compositions, the ink is kept at a temperature lower than the ejection temperature in the state of a stand-by of hot-melt ink-jet printers, and must be heated up to the ejection temperature before the printing is started after the signals are received, thus it has taken a time.
None of the above hot-melt solid ink compositions have a good adhesion to printing mediums such as paper and OHP (overhead projector) sheets while retaining light transmission properties (transparency) of the ink itself after the ink has once melted, formed printed images and solidified. For example, the conventional ink compositions have a problem that, when images are printed OHP sheets that transmit light and have a poor ink permeability, the ink may crystallize in the course where it is cooled to solidify, so that the ink itself results in lack of light transmission properties and also the ink may come off when the OHP sheets are bent or rubbed with the hand.